The purpose of the present work is to demonstrate the reliability of a digital image processing technique. A CCD (Charge Coupled Device) is used to perform an investigation on the bubble properties of a 2D gas-solid fluidized bed, and the aim of this study is to establish a criterion in order to reject all images that could not be interpreted as bubbles. This may lead to an alteration of the information obtained. The two-dimensional fluidized bed is 20 cm wide and at least 20 cm high, and the field of view of the camera is 12.54 cm wide and 12.99 cm high. In this way, bubbles could be totally or partially placed into this field of view so that a partial image of a bubble could be analyzed as a whole bubble. With this calibration, a geometric decision criterion for the rejection of nonbubble images was developed.